The present invention relates to a solid-state image sensing device and, more particularly, to a double-layered (multilayered) solid-state image sensing device in which a photoconductive film, acting as a photoelectric converting section, is stacked on a substrate.
A recently developed solid-state image sensing device has a structure in which a photoconductive film, acting as a photoelectric converting section, is stacked on a chip substrate. This type of device is known as a "multilayered solid-state image sensing device" to those skilled in the art. The chip substrate is comprised of a silicon single-crystal substrate on which a cell matrix, consisting of a plurality of pixels, is formed. A signal charge storing section and a signal charge readout section, which define each pixel, are formed in the surface portion of the substrate. When the photoconductive film is stacked on the substrate, the cell opening ratio can be increased, thereby improving cell sensitivity and suppressing blooming.
A photoconductive film stacked on a substrate of an image sensor is normally made of a high-resistance semiconductor material (e.g., amorphous silicon). An amorphous silicon photoconductive film has more traps than a normal monocrystalline film. For this reason, after the image sensor is irradiated with image light, charges trapped in the photoconductive film tend to be thermally released. When signal charges are read out, if charges trapped in the photoconductive film over a plurality of field images are released, these charges become mixed in with an image signal, as a residual output, thereby causing an "after-image". As a result, reproduced image quality is degraded. This problem is inherent in doublelayered (multilayered) solid-state image sensing devices, and limits the practical application range of these devices.
In order to suppress the above after-image phenomenon, it is generally understood that the number of traps which are inherently present in the photoconductive film must be decreased. However, the number of traps relates directly to the material comprising the amorphous silicon film, and it is very difficult to greatly reduce this number.
In another method for reducing the number of traps in the photoconductive film, the film is irradiated with bias light, to fill the traps with bias charges. When the traps are filled with bias charges, since the photoconductive film can no longer trap charges, it is converted into a virtually inactive film. In this case, however, when signal charges are read out, the bias charges trapped in the photoconductive film may often be read out together with effective signal charges produced upon irradiation of image light. If the bias charges are read out together with the effective signal charges, this causes the amount of bias charges in the image sensor to be unstable, and noise components contained in an image increase. As a result, the signal-to-noise (S/N) ratio of a reproduced image is degraded. Therefore, although a solution to the problem of preventing an after-image in a multilayered solid-state image sensing device has been greatly desired, it has not yet been found.